The invention relates to a method of depositing microcrystalline solid particles from a vapor phase containing a carbonaceous gas chemical vapor deposition (CVD), in which the solid particles to be deposited are deposited at a pressure in the range from 10.sup.-5 to 1 bar onto a substrate which has been heated to a temperature in the range from 450.degree. to 1200.degree. C. and in which the chemical vapor phase reactions are effected in the gas phase which is thermally excited by means of a resistance heat conductor.
The deposition of micro-crystalline solid material from the gas phase using chemical vapor deposition is used for a wide range of applications for producing self-supporting preforms or also for forming coats on different types of substrates.
Generally, it is a characteristic of a CVD-process that a mixture of gases interacts with a substrate surface at a relatively high temperature while decomposing at least one gas forming part of the gas mixture and the depositing of a reaction product in the solid state on the upper surface of the substrate. The conventional CVD-process requires high temperatures, reactive gases and an arrangement suitable to perform such a method. Typical process parameters are temperatures in the range from 450.degree. to 2500.degree. C., pressures in the range from 10.sup.-5 to 1 bar and a process gas mixture of at least one reactive gas and additional gases such as inert, oxidizing or reducing gases.
The reaction product (solid particles) are correspondingly obtained by
______________________________________ pyrolysis (for example: CH.sub.3 SiCl.sub.3 .fwdarw. SiC + 3HCl) or reduction (for example: WF.sub.6 + 3H.sub.2 .fwdarw. W + 6HF) or oxidation (for example: SiH.sub.4 + O.sub.2 .fwdarw. SiO.sub.2 + 2H.sub.2) or co- (for example: reduction TiCl.sub.4 + 2BCl.sub.3 + 5H.sub.2 .fwdarw. TiB.sub.2 + ______________________________________ 10HCl).
Energizing the process gas mixture within the reactor is effected in an excitation zone (gas phase range with maximum energy content), which, for example, be formed over a plasma, can he produced by coupling-in microwave or high frequency energy or energy from an electric d.c. voltage field, over a wire heated by current flow or over a flame. In the excitation zone the gas phase reactions are stimulated and subsequently there occurs a precipitation of the reaction products onto the substrate which preferably is positioned at some distance from the excitation zone, which substrate is arranged on a substrate holder which is temperature-controlled in a conventional manner. The residual gases are removed from the reactor by means of a vacuum pump.
Jap. J. Appl phys. 21 (1982), Nr. 4, pages L183 to L185 schematically shows in FIG. 1 an arrangement for depositing diamond particles using CVD from a methane/hydrogen vapor phase, in which the reaction gas mixture is energized by a resistance heat conductor constituted by a tungsten filament heated to a temperature of approximately 2000.degree. C.
In trials performed within the framework of the invention for the deposition of polycrystalline diamond layers using a CVD-process it was found that the material of the resistance heat conductor (preferably molybdenum, tungsten or tantalum) becomes brittle under process conditions (hydrocarbon gas/hydrogen-atmosphere, temperatures over 2000.degree. C.) within a very short period of time (after approximately 10 h process duration) and spontaneously break when they are slightly shaken or touched. The Ep-A2 272 418 discloses, for example, such a method and an arrangement suitable therefor. Therein, using a thermal CVD-process diamond particles are deposited from a carbon/hydrogen atmosphere onto a substrate which is heated to a temperature in the range from 1000.degree. to 2500.degree. C. and consists of a high melting metal or carbon, the reaction gas being excited by means of a tungsten heat conductor having a temperature of 2000.degree. C.
Because it is so simple, this method, in which the gas phase is excited via a heated resistance wire (alternatively denoted "filament method" in the relevant literature), is at present one of the most widely used processes for producing diamond coats. The linear rate of growth is in the order of magnitude of 1 .mu.m/h, when methane is used as the carbonaceous gas. The deposition rate can be increased by approximately one order of magnitude when other organic compounds are used as carbon suppliers for the reaction gas mixture, for example, alcohols, ethers, ketones, amines. However, it was found in these processes that a continuous process duration of approximately 10 h can hardly be exceeded, when the ordinarily used resistance heat conductors of high-melting metal such as tungsten, molybdenum and tantalum are employed. This is because these conductors become embrittled (as has been described) during this process duration.
This embrittlement is caused by the fact that the metals of the resistance heat conductor chemically react with the reactive gas phase, for example a forming carbide, and the attendant structural changes in the resistance heat conductor then result in unwanted mechanical instabilities. Consequently, when resistance heat conductors made of high-melting metals are used it is recommended to exchange them after a relatively short process duration, which means that the deposition procedure must be interrupted and it is then hardly possible to produce homogeneous solid particle layers of a greater thickness ( &gt;100 .mu.m) in a continuous process or to deposit a plurality of thin solid particle layers one after the other without exchanging the resistance heat conductor.
The invention has for its object to improve the method defined in the opening paragraph for depositing micro-crystalline solid particles from a gas phase containing carbonaceous gas by means of CVD to such an extent that a longer continuous process duration can be achieved, without the necessity of exchanging the resistance heat conductor.
According to the invention, this object is accomplished in that a resistance heat conductor made of a carbide of at least one transition metal of the secondary group IVa to VIa of the periodic table of the elements (PTE) with a carbon content which substantially corresponds to a stoichiometrical composition of the carbide and having a melting point of &gt;2000.degree. C. is used.